In general, semiconductor integrated circuits are vulnerable to electrostatic discharge, and thus may be easily broken. One typical generation source of the electrostatic discharge is, for example, a human body in which static electricity more than 2,000 V is accumulated. Thus, when a human being handles an IC package on which a semiconductor integrated circuit is mounted without taking any protective measure for static electricity, there is a possibility that abrupt electrostatic discharge occurs, which may cause physical breakage to respective circuits and respective devices which constitute the semiconductor integrated circuit.
As a method for protecting the semiconductor integrated circuit from such electrostatic discharge, it is common to form a resistive route by inserting a diode as an electrostatic protective device to shunt a route to an electrode pad from the semiconductor integrated circuit that is a circuit to be protected. With this method, it is possible to prevent the electrostatic discharge from reaching the circuit to be protected.
On the other hand, recently, with higher-speed wireless communication in mobile information communication terminals, etc., devices that handle a high-frequency RF signal have become increasingly important. Along with miniaturization, a silicon on insulator (SOI) technique and a silicon on sapphire (SOS) technique have been indispensable for higher-speed devices. According to the SOI technique and the SOS technique, an insulating separation in a depth direction is also performed in addition to a device separation in an in-plane direction, thus allowing for a decrease in a parasitic capacitance due to shrinkage of a junction area. Thus, it is possible to achieve enhancement of RF characteristics such as high-speed operation, decrease in harmonic distortion, and enhancement of isolation characteristics indicating leakage in high-frequency signal transmission.
Along with such enhancement of the characteristics of the device, an influence of the electrostatic protective device on the RF characteristics has also become important. A case is considered in which a diode is inserted, as the electrostatic protective device, for example, between an RF signal line and a reference potential line, in order to protect, from the electrostatic discharge, an internal circuit to be connected to an input/output terminal that handles the RF signal. In this case, it is necessary for the diode as the electrostatic protective device to have a junction area from which a considerable electrostatic discharge capacity is obtained in terms of securing electrostatic resistance. On the other hand, the parasitic capacitance in proportion to the junction area possessed by the diode affects adversely to the RF characteristics such as the harmonic distortion and isolation characteristics. Thus, it has become difficult to achieve both the electrostatic resistance and the RF characteristics, because securing the electrostatic discharge capacity and the decrease in the parasitic capacitance are contradictory to each other.
In an effort to achieve both the electrostatic resistance and the RF characteristics, there have been proposed, for example, a semiconductor integrated circuit disclosed in PTL 1and an electrostatic discharge protective device disclosed in PTL 2.